The present invention relates to a RFID tag and manufacturing method thereof, and particularly to a RFID tag comprising a loop antenna and manufacturing method thereof, and takes advantage of characteristics as a tag antenna even when attached to a conductive body or non-conductive body.
Conventionally, in the distribution industry, the express business industry and the like, a method of printing or attaching a barcode to a product itself or to the product packaging, and reading the barcode with a barcode reader has been widely used as a method for managing individual product information. However, when reading a barcode in the related barcode processing method, the barcode had to be in contact with the barcode reader, which makes the work of reading barcodes troublesome. Also, in the prior barcode processing method, there was a problem in that it was not possible to add new information or update information to the barcode itself.
Therefore, recently there is a demand, and methods are being put into practical use of attaching a RFID (Radio Frequency Identification) tag to products in the place of a barcode and reading product information wirelessly (electromagnetic coupling) without contact. A RFID tag is a tag to which a radio information communication function has been added to an IC card function, and comprises a nonvolatile memory capable of recording information, however does not have a battery (power supply unit). Therefore, when a tag reader reads information from the RFID tag memory without contact, power is supplied to the RFID tag by electromagnetic waves, and information is read from that memory. Depending on the RFID tag, it is possible to greatly improve operability, and by using technology such as a verification function for the RFID tag, encryption and the like, it is possible to ensure excellent security.
FIG. 18 is a drawing explaining a RFID tag, where a reader 1 transmits a radio signal (electromagnetic waves) modulated by transmission data from an antenna 2 to a RFID tag 3. The antenna 3a of the RFID tag 3 inputs the received signal to a rectifier circuit 3b and a modulation and demodulating (modem) circuit 3c. The rectifier circuit 3b converts the radio signal to a direct-current voltage, and supplies the result to the modem circuit 3c and logic circuit 3d, and operates as a power supply. The modem circuit 3c demodulates control data that is transmitted from the reader 1, and inputs it to the logic circuit 3d. The logic circuit 3d performs logical processing according to that control data (command), and reads information stored in the internal memory, for example, and inputs it to the modem circuit 3c. The modem circuit 3c uses the information that was input from the logic circuit to modulate the carrier wave, and transmits the result from the antenna 3a to the reader 1.
Various types of RFID tags have been proposed. One type is a dielectric base sheet such as a plastic or paper sheet on which an antenna pattern for radio communication and IC chip (LSI) are mounted. By attaching the RFID tag to a non-conductive body, required performance for communication distance and the like is obtained. However, when the RFID tag is attached to metal such as steel, the metal impairs the communication waves of the RFID tag, and problems occur such as a decrease in communication distance.
FIG. 19 is a drawing explaining the occurrence of trouble, where (A) of FIG. 19 is a drawing explaining the case in which a RFID tag comprising a half-wave dipole antenna pattern DP is attached to a non-conductive body (not shown in the figure), and the power (open voltage V) required for the IC chip is generated in the dipole antenna DP by the radio waves emitted from a reader/writer antenna. Also, it is possible to run a current I through the dipole antenna DP and transmit electromagnetic waves toward the reader/writer antenna.
However, when a RFID tag comprising a dipole antenna DP is attached to a metal body, the tangential component of the electric field on the metal surface becomes 0 from the boundary conditions, and the surrounding electric field becomes 0. Therefore, it is not possible to supply the necessary power for the IC chip of the RFID tag. Also, it is not possible to transmit (scatter) electromagnetic waves from the tag antenna toward the reader/writer antenna. In other words, as shown in (B) of FIG. 19, when a RFID tag comprising a dipole antenna pattern DP is attached to a metal body MTL, by running a current I through the dipole antenna DP, a current image IMG is generated having current that flows in the opposite direction in the metal body MTL due to mirror image theory. This current image negates the electromagnetic field generated by the dipole antenna current I, and it is not possible to supply the necessary power to the IC chip of the RFID tag, thus it becomes impossible to transmit electromagnetic waves from the tag antenna toward the reader/writer antenna. Therefore, a RFID tag comprising a tag antenna capable of transmitting and receiving electromagnetic waves without causing a loss in antenna gain when attached to a metal surface is desired.
Therefore, as shown in (C) of FIG. 19, reducing the effect of the current image by increasing the distance D from the surface of the metal body MTL to the dipole antenna pattern DP has been considered, however, the thickness of the RFID tag increases, which poses a problem when using the tag. Also, a UHF bandwidth RFID system has the advantage of a long communication distance when compared with that of other frequency bandwidths; however, a UHF bandwidth dipole tag antenna requires normally the length of a half wave (approximately 16 cm). This length is maintained by attaching the tag antenna to a dielectric body or by bending it, and it is miniaturized, however, the bandwidth becomes narrow. From the above problems, desired is an RFID tag that comprises a tag antenna having as wide a bandwidth as possible, and whose antenna gain is not lost even when the RFID is miniaturized and made as small as possible.
Also, in order to efficiently supply the receiving power of the tag antenna to an LSI chip, it is necessary to match the impedance of the tag antenna with that of the LSI chip. In order to do that, an impedance conversion circuit is necessary, however, that increases the manufacturing cost of the RFID tag. Therefore, it is necessary to match the tag LSI chip with the tag antenna without using an impedance conversion circuit. In other words, desired is an RFID tag that comprises a tag antenna for which impedance matching is established even when an impedance conversion circuit is not used.
A prior RFID tag comprising a dipole antenna had the problem of decreased communication distance when attached to metal as described above. Therefore, various metal-compatible tag antennas have been developed for the UHF bandwidth (refer to JP2002-298106 A), however all of them were large having a thickness of 4 mm or thicker and a length of 10 cm or longer.
Therefore an RFID tag comprising a small antenna capable of transmitting and receiving electromagnetic waves even when attached to a metal surface is proposed (refer to JP2006-53833 A). As shown in FIG. 20, this proposed RFID tag 10 comprises: a rectangular-shaped dielectric member 11, an antenna pattern 12 for transmitting and receiving that is provided on the surface of the dielectric member 11 and forms a loop antenna, and an IC chip 15 that is electrically connected to the antenna pattern 12 by way of a chip-mounting pad 13.
As the rectangular-shaped dielectric member 11, it is possible to use a board made from plastic having a specified dielectric constant and containing glass and the like, or in other words, a so-called high-frequency board. The antenna pattern 12 on the flat portion of the dielectric member 11 is formed by etching a conductor (for example, a metal conductor such as copper). Also, a pair of chip-mounting pads 13 for electrically connecting the IC chip 15 to the antenna pattern 12 are formed at the same time as the antenna pattern 12 by the aforementioned etching. Furthermore, the antenna pattern 12′ on the side surface portions (portions that form the thickness) of the dielectric member 11 is formed by a well known so-called side surface conduction method using plating. The IC chip 15 comprised a communication circuit for writing and reading information without contact, a memory, and a specified control circuit, as well as it comprises chip electrodes (not shown in the figure) for electrically connecting it to the chip-mounting pads 13 that extend to the antenna pattern 12. As shown in FIG. 21, the RFID tag 10 that is constructed as described above is attached to a metal body MTL using an insulating adhesive (for example double-sided tape) 16, and it is covered by a protective film or the like (not shown in the figure). This RFID tag 10 can also be constructed so that it comprises beforehand the aforementioned adhesive 16 for attaching it to the product to be used, and the protective film.
With this RFID tag having this kind of loop antenna construction, by letting a current I flow through the loop antenna, an current image IMG as shown in FIG. 22 is generated on the metal body MTL due to the same mirror image principle as shown in (B) of FIG. 19. However, in this loop antenna, only the current on the bottom of the RFID tag is negated by this current image IMG, and equivalently a current I′ as shown by the dot-dash line is considered to flow, so it is possible to supply the necessary power to the LSI chip 15 of the RFID tag, and to transmit electromagnetic waves from the loop antenna toward a reader/writer antenna.
To manufacture a RFID tag with the loop antenna construction described above, first, the surfaces of the printed circuit board (dielectric member) 11, both surfaces of which are covered by a conductor, are etched to form the antenna patterns shown in FIG. 20, then plating is performed on the side surfaces, and the antenna pattern on the top surface is connected with the antenna pattern on the bottom surface by the plating 12′ to form a loop antenna (loop antenna formation process). Next, the IC chip 15 is mounted in order to electrically connect the chip electrodes of the IC chip 15 to the aforementioned chip-mounting pads 13. To mount the chip, a mounting technique such as so-called flip-chip mounting can be employed.
FIG. 23 shows a different manufacturing method in which the antenna pattern 12 and chip-mounting pads 13 are printed on insulating film 20, and as shown in FIG. 24, that film is wrapped around the dielectric member 11 and fastened, then the IC chip is mounted on the chip-mounting pads 13.
According to the prior RFID tag described above having loop antenna construction, it is possible to transmit and receive electromagnetic waves even when attached to a metal surface regardless of whether the tag is small or thin, it is possible to lengthen the communication distance, it is possible to keep the gain nearly constant over a wide range, and it is possible to perform impedance matching even without an impedance conversion circuit.
However, in the first manufacturing method of the prior RFID tag, a printed circuit board is used, and a complex processes such as an etching process and side-surface plating process are necessary, so there was a problem in that the manufacturing cost increased. Also, in the second manufacturing method of the prior RFID tag, the IC chip 15 had to be mounted after wrapping insulating film around the dielectric member 11, and when wrapping and fastening the insulating film around the dielectric member, high precision is required when positioning the wrapping. In other words, the IC chip is very small, for example 0.4 mm square, so in order to mount it properly, high precision, for example positioning precision of 10 to several 100 μm is necessary when wrapping the insulating film around the dielectric member 11.